All right, if that's the case, then you should be able to take an image taken in a fairly dark area, like , say, a basement room with the curtains closed (or mostly closed if they are blackout curtains. Set your camera to ISO 12800, using F3.5 and 1/30-1/60 second shutter speed, take a picture, it's probably going to be dark, and that's fine, actually if it's not, then take it again with a faster shutter soeed, then change the iso to 100, use the same aperature and shutter speed, take another picture. Now, according to you, simply by adjusting the brightness in your photo editing program, you should be able to adjust the iso 100 picture and make both pictures look identical... In theory sure, but you can't. Oh, you might get it close, depending on lighting, but identical? Nope. Give it a try. Like, actually do it, and see what happens. Lots of people shoot in jpeg only, and since you don't make the distinction, give it a try in jpeg too. Spoiler, the results aren't nearly as good as the imperfect results you got with raw.

Question. Do you shoot everything in your cameras native iso then just adjust brightness in post? Because if not, if you ever adjust iso in your camera, you are a hypocrite telling people something doesn't exist then using it yourself.

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What the hell are you looking at? Are you looking at the jpeg image that the camera has produced? If so, can you tell me what mechanisms and firmware have been used to produce that image? Additionally, can you tell me what parameters each has used to generate that image?

1/2000 @ f/8 ISO 900 on a bright sunny day was plenty fast enough to freeze the blades of a helicopter. Sorry for not joining in on the argument, i have to go to work. So i'll just answer the OP's question and move on lol